While solid fuel boosters do have their problems, they're far cheaper to build than liquid boosters, have a better ISP for their weight / volume and contain almost no hard or expensive parts, like turbopumps and electronics, (mostly a nozzle and ignitors) so they're very expendable.

The main problem with building the first three stages (0, 1 and 2) all out of solid fuel boosters is, that you have much less control: you can only fire them, no throttling or relighting, and steering is only possible with vanes or other rocket engines.

Then again, that's only a problem if you want to follow an exact trajectory, no matter the weather, weight of the payload, or the insertion vector of it. Which the third (and possibly fourth) stage should handle in any case.

So, why not build a solid first stage, strap some solid boosters to it, put a solid second stage on top, and tell your clients: "With that payload weight, we'll insert you at about this vector, plus or minus so much. If you want to reach that vector, it requires a third stage of these specifications."

Simple and cheap, fire and forget.

Much of the steering can be done with small solid packs as well, as long as you don't care if it is very exact. You can counter the wind for the first part, and make some rough corrections later. If you use "multi-barrel" steering pods, you fire the amount required.

Ok, you need a liquid fuel, third stage engine that is a bit beefier than normal for final injection, but you save loads of weight and money on everything else, not in the least the organization around it: no fueling needed, very little flight control, etc.

I believe this is what ESA is doing at the moment with their "Vega" launcher, not sure how they steer the thing or know what the nitty gritty details are but I guess well find out soon enough when they launch it.

the impression that to make everything reusable seems to cost more than it saves is misleading and no real fact.

There are threads about this in the Financial Barriers section. The critical point is the minimum number of launches a vehicle will do really. Such a number of launches might be planned or sold or the like.

Also reliability might help to get a number.

The cost you seem to talk about are the investment costs - those are no vaid criterion since they would have to be divided by the number of launches and since the costs of the fuel need to be involved. A look to the section mentioned above might clarify this.

the impression that to make everything reusable seems to cost more than it saves is misleading and no real fact.

I think that depends a lot on many things.

For example, if salvaging and refurbishing a simple solid-fuel booster costs more than building a new one (which is very likely, as it's mostly a hollow cylinder with a nozzle and ignitor), you're not going to save money. And to be able to salvage it, you need to add at least a parachute, have to be able to transport it back to base, need additional equipment (weight) to have it land at an accessible location, without damaging it (reinforcements and/or some kind of landing gear / shock absorbers).

While that might make sense for some parts, it doesn't for cheap things like solid-fuel boosters and stages.

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There are threads about this in the Financial Barriers section. The critical point is the minimum number of launches a vehicle will do really. Such a number of launches might be planned or sold or the like.

Also reliability might help to get a number.

The cost you seem to talk about are the investment costs - those are no vaid criterion since they would have to be divided by the number of launches and since the costs of the fuel need to be involved. A look to the section mentioned above might clarify this.

Dipl.-Volkswirt (bdvb) Augustin (Political Economist)

If you want to get through those financial barriers, your best bet is to make it as simple and cheap as possible: less can go wrong, and you can get hardware off the ground (to get the funding you require) for less money.

And with simple and cheap stages and boosters, salvaging and refurbishing are no options; for reusability and quick turnaround you require the parts to fly back to base by themselves. Or, in other words: you need liquid propellants (which make for much more expensive stages than solid-fuel ones), and you need to add a lot of extra electronics (which are expensive and can fail), fuel (which reduces the weight of the payload) and R&D.

Btw, I think that a completely solid-fuel rocket (except for the last stage) is the way to go, until we manage SSTO. And I would add cheap and expendable boosters to that SSTO as well, to make it work.

Its all very well talking about reusability but you have to look at performance and thus why we tend to lean towards more liquid propellants.
They have a lower ISP than liquids and if your are leaning towards them for a efficient design you'd be going in the wrong direction as you would be hard pressed to get a large mass ratio, considering the entire casing has to be made to withstand the combustion pressures, adding weight.
Add in the fact that they can't be throttled (within reason) and easily shut down (not too common), these can be done but adds hugely to your development budget.

Theres no reason why you cant spend that little bit extra in developing that liquid booster that can be recovered but yet could have better performance, throttling capability (dependant of flight), and more features etc.

As a strap on booster they are great but an entire launch vehicle, not saying its bad but you really have to weigh up your options.

Btw adding expendable boosters to a SSTO defeats the purpose of an SSTO, if I get right what your saying,like saying the shuttle is an SSTO, one stage to orbit, thats all it can be. (Correct me if I have read you wrong)

Sorry, guys cant hear yah! Your going to have to speak up! These cheap easy to build solid motors make to much noise! People! No, we dont fly people on solid rockets just hardware!
Just teasin guy's but there is more than just liquid on the stove here. We are Meat and Potatoes guy's At Prometheus and somtin's cookin over here for a change.

Its all very well talking about reusability but you have to look at performance and thus why we tend to lean towards more liquid propellants.They have a lower ISP than liquids and if your are leaning towards them for a efficient design you'd be going in the wrong direction as you would be hard pressed to get a large mass ratio, considering the entire casing has to be made to withstand the combustion pressures, adding weight. Add in the fact that they can't be throttled (within reason) and easily shut down (not too common), these can be done but adds hugely to your development budget.

But you can make solid rocket casings out of composites, which cannot be done with cryogenic tanks. The fuel itself acts as an insulated container.

And while you cannot throttle directly, you can design the thrust at each part of the burn time by the pattern of the inner chamber, and by layering different fuels.

Restarting can be done at predetermined times through staging.

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Theres no reason why you cant spend that little bit extra in developing that liquid booster that can be recovered but yet could have better performance, throttling capability (dependant of flight), and more features etc.

As a strap on booster they are great but an entire launch vehicle, not saying its bad but you really have to weigh up your options.

Reusability can be a goal by itself, for other reasons than cost effectiveness. If you want cheap, discardable solid-fuel stages and boosters are probably the best bet.

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Btw adding expendable boosters to a SSTO defeats the purpose of an SSTO, if I get right what your saying,like saying the shuttle is an SSTO, one stage to orbit, thats all it can be. (Correct me if I have read you wrong)

Iain

Yes, a shuttle is such a hybrid as well. And about as reusable as we got so far. But cost-effective?